This invention relates to a light-emitting semiconductor device, or light-emitting diode according to more common parlance, and more particularly to such devices having active layers made from chemical compounds such for example as aluminum gallium arsenide (AlGaAs), aluminum gallium indium phosphide (AlGaInP), and gallium nitride (GaN). The invention also concerns a method of making such light-emitting devices.
Compound semiconductors containing AlGaInP, for instance, represent familiar materials of light-emitting semiconductor devices. An example of such device has a substrate of gallium arsenide (GaAs) on which there are laminated a plurality of active semiconductor layers composed primarily of AlGaInP. The active semiconductor layers for generating light include an n-type semiconductor layer, an active layer and a p-type semiconductor layer. The AlGaInP semiconductor layers are relatively easy to grow on the GaAs substrate by epitaxy.
One of the problems encountered with this conventional light-emitting device is that the GaAs substrate is highly absorptive of the light in the wavelength range emitted by the active semiconductor layers. Much of the light that has issued from the active semiconductor layers toward the substrate has been absorbed thereby, running counter to the objective of making the light-emitting device as high as feasible in efficiency.
A known remedy to this problem was to remove the GaAs substrate following the epitaxial growth of the active semiconductor layers thereon. A transparent support substrate of gallium phosphide (GaP) or the like, different from the removed growth substrate which had been used for epitaxial growth of the active layers, was then bonded to the active semiconductor layers. Then a reflective electrode was formed on the support substrate. This remedy proved unsatisfactory, however, as the active semiconductor layers and the transparent support substrate gave rise to electrical resistance at the interface therebetween. This resistance made the forward voltage between the anode and cathode of the light emitting device inconveniently high.
A solution to this weakness is found in Japanese Unexamined Patent Publication No. 2002-217450. This prior application teaches the creation of thin, isolated regions of gold-germanium-gallium (Au—Ge—Ga) alloy on the underside of the active semiconductor layers. The Au—Ge—Ga alloy is favorable from the standpoint of ohmic contact but not very reflective. The isolated alloy regions as well as the exposed surface of the active semiconductor layers were therefore covered with a layer of aluminum or like reflective metal. To this reflective layer was bonded a support substrate of electrically conductive silicon or like material.
The aluminum layer makes no satisfactory ohmic contact with the exposed surface of the active semiconductor layers. But the Au—Ge—Ga alloy regions do make good ohmic contact with both the active semiconductor layers of AlGaInP or the like and the aluminum layer. The result was a reduction in the forward voltage between anode and cathode. The aluminum layer is inherently capable of reflecting much of the light coming from the active semiconductor layers toward the support substrate.
However, this second recited prior art device also proved to have its own weaknesses. One of these weaknesses arose in conjunction with the manufacturing process of the device, which involved several heat treatments. Undesired reactions took place as a result of such heat treatments between the reflective metal layer, the Au—Ge—Ga alloy regions, and the neighboring part of the active semiconductor layers. The result was a diminution of reflectivity at their interfaces. High-efficiency light-emitting devices were therefore not obtainable with as high a yield as had been expected.
Another weakness concerned the isolated regions of Au—Ge—Ga alloy on the underside of the active semiconductor layers. These isolated alloy regions made it difficult to fulfill both of the objectives of higher efficiency of light emission and the reduction of the forward voltage. The greater the Au—Ge—Ga alloy regions were made in size with respect to the surface area of the active semiconductor layers for better ohmic contact, the lower could be the forward voltage, but at the same time the less was the efficiency of light emission. This reduction of efficiency came from less reflection of light from the active semiconductor layers by the aluminum layer because of the isolated regions of larger size interposed therebetween. Thus, as far as this prior art device was concerned, the fulfillment of the noted dual objective was a self-contradictory requirement.